Chronic hypoxia induces constitutive p38 mitogen-activated protein kinase activity that correlates with enhanced cellular proliferation in fibroblasts from rat pulmonary but not systemic arteries

Connexin 43 plays a role in proliferation and migration of pulmonary arterial fibroblasts in response to hypoxia

Pulmonary hypertension (PH) is a disease associated with vasoconstriction and remodelling of the pulmonary vasculature. Pulmonary artery fibroblasts (PAFs) play an important role in hypoxic-induced remodelling. Connexin 43 (Cx43) is involved in cellular communication and regulation of the pulmonary vasculature. Using both in vitro and in vivo models of PH, the aims of this study were to (i) investigate the role of Cx43 in hypoxic-induced proliferation and migration of rat PAFs (rPAFs) and rat pulmonary artery smooth muscle cells (rPASMCs) and (ii) determine whether Cx43 expression is dysregulated in the rat sugen5416/hypoxic model of PH. The role of Cx43 in hypoxic-induced proliferation and migration was investigated using Gap27 (a pharmacological inhibitor of Cx43) or genetic knockdown of Cx43 using siRNA. Cx43 protein expression was increased by hypoxia in rPAFs but not rPASMCs. Hypoxic exposure, in the presence of serum, resulted in an increase in proliferation of rPAFs but not rPASMCs. Hypoxic exposure caused migration of rPAFs but not rPASMCs. Phosphorylation of p38 mitogen-activated protein kinase (MAPK) and ERK1/2 were increased by hypoxia in rPAFs. The effects of hypoxia on proliferation, migration and MAPK phosphorylation in rPAFs were attenuated in the presence of Gap27 or Cx43 siRNA. Cx43 protein expression was increased in sugen5416/hypoxic rat lung; this increased expression was not observed in sugen5416/hypoxic rats treated with the MAPK pathway inhibitor GS-444217. In conclusion, Cx43 is involved in the proliferation and migration of rPAFs in response to hypoxia via the MAPK signalling pathway.

Pirfenidone Inhibits Hypoxic Pulmonary Hypertension through the NADPH/ROS/p38 Pathway in Adventitial Fibroblasts in the Pulmonary Artery

Hypoxic pulmonary hypertension (HPH) is a devastating disease characterized by progressive vasoconstriction and vascular remodeling. Pirfenidone (PFD) inhibits the progression of HPH, though the molecular mechanisms remain unknown. This study is aimed at determining the role and mechanism of PFD in HPH in human pulmonary artery adventitial fibroblasts (HPAAFs), which were cultured under normal or hypoxic conditions. NOX4 and Rac1 were inhibited or overexpressed by shRNA or pcDNA3.1, respectively. Proliferation of HPAAFs was quantified by colorimetric 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium (MTT) assays to assess cellular metabolic activity, cell counts, and ethynyldeoxyuridine (EdU) assays to detect DNA synthesis. Migration of HPAAFs was assessed by a wound healing assay. The expression levels of smooth muscle alpha-actin (a-SMA) and procollagen I (COL1A1) were assessed by RT-PCR and western blot analysis. PFD suppressed hypoxia-induced proliferation and migration of HPAAFs. Compared with the hypoxic control group, PFD reduced the expression of a-SMA and procollagen I (COL1A1). PFD reduced hypoxia-induced phosphorylation of p38 through the NOX4/reactive oxygen species (ROS) signaling pathway. Moreover, Rac1 also decreased hypoxia-induced phosphorylation of p38, without any cross-interaction with NOX4. These findings demonstrate that PFD is a novel therapeutic agent to prevent cell proliferation, migration, and fibrosis, which might be useful in inhibiting vascular remodeling in patients with HPH.

Apoptosis signal-regulating kinase 1 inhibition in in vivo and in vitro models of pulmonary hypertension

Pulmonary arterial hypertension, group 1 of the pulmonary hypertension disease family, involves pulmonary vascular remodelling, right ventricular dysfunction and cardiac failure. Oxidative stress, through activation of mitogen-activated protein kinases is implicated in these changes. Inhibition of apoptosis signal-regulating kinase 1, an apical mitogen-activated protein kinase, prevented pulmonary arterial hypertension developing in rodent models. Here, we investigate apoptosis signal-regulating kinase 1 in pulmonary arterial hypertension by examining the impact that its inhibition has on the molecular and cellular signalling in established disease. Apoptosis signal-regulating kinase 1 inhibition was investigated in in vivo pulmonary arterial hypertension and in vitro pulmonary hypertension models. In the in vivo model, male Sprague Dawley rats received a single subcutaneous injection of Sugen SU5416 (20 mg/kg) prior to two weeks of hypobaric hypoxia (380 mmHg) followed by three weeks normoxia (Sugen/hypoxic), then animals were either maintained for three weeks on control chow or one containing apoptosis signal-regulating kinase 1 inhibitor (100 mg/kg/day). Cardiovascular measurements were carried out. In the in vitro model, primary cultures of rat pulmonary artery fibroblasts and rat pulmonary artery smooth muscle cells were maintained in hypoxia (5% O₂) and investigated for proliferation, migration and molecular signalling in the presence or absence of apoptosis signal-regulating kinase 1 inhibitor. Sugen/hypoxic animals displayed significant pulmonary arterial hypertension compared to normoxic controls at eight weeks. Apoptosis signal-regulating kinase 1 inhibitor decreased right ventricular systolic pressure to control levels and reduced muscularised vessels in lung tissue. Apoptosis signal-regulating kinase 1 inhibition was found to prevent hypoxia-induced proliferation, migration and cytokine release in rat pulmonary artery fibroblasts and also prevented rat pulmonary artery fibroblast-induced rat pulmonary artery smooth muscle cell migration and proliferation. Apoptosis signal-regulating kinase 1 inhibition reversed pulmonary arterial hypertension in the Sugen/hypoxic rat model. These effects may be a result of intrinsic changes in the signalling of adventitial fibroblast.

ASK1 Inhibition Halts Disease Progression in Preclinical Models of Pulmonary Arterial Hypertension

RATIONALE

Progression of pulmonary arterial hypertension (PAH) is associated with pathological remodeling of the pulmonary vasculature and the right ventricle (RV). Oxidative stress drives the remodeling process through activation of MAPKs (mitogen-activated protein kinases), which stimulate apoptosis, inflammation, and fibrosis.

OBJECTIVES

We investigated whether pharmacological inhibition of the redox-sensitive apical MAPK, ASK1 (apoptosis signal-regulating kinase 1), can halt the progression of pulmonary vascular and RV remodeling.

METHODS

A selective, orally available ASK1 inhibitor, GS-444217, was administered to two preclinical rat models of PAH (monocrotaline and Sugen/hypoxia), a murine model of RV pressure overload induced by pulmonary artery banding, and cellular models.

MEASUREMENTS AND MAIN RESULTS

Oral administration of GS-444217 dose dependently reduced pulmonary arterial pressure and reduced RV hypertrophy in PAH models. The therapeutic efficacy of GS-444217 was associated with reduced ASK1 phosphorylation, reduced muscularization of the pulmonary arteries, and reduced fibrotic gene expression in the RV. Importantly, efficacy was observed when GS-444217 was administered to animals with established disease and also directly reduced cardiac fibrosis and improved cardiac function in a model of isolated RV pressure overload. In cellular models, GS-444217 reduced phosphorylation of p38 and JNK (c-Jun N-terminal kinase) induced by adenoviral overexpression of ASK1 in rat cardiomyocytes and reduced activation/migration of primary mouse cardiac fibroblasts and human pulmonary adventitial fibroblasts derived from patients with PAH.

CONCLUSIONS

ASK1 inhibition reduced pathological remodeling of the pulmonary vasculature and the right ventricle and halted progression of pulmonary hypertension in rodent models. These preclinical data inform the first description of a causal role of ASK1 in PAH disease pathogenesis.

Mechanisms contributing to persistently activated cell phenotypes in pulmonary hypertension

Chronic pulmonary hypertension (PH) is characterized by the accumulation of persistently activated cell types in the pulmonary vessel exhibiting aberrant expression of genes involved in apoptosis resistance, proliferation, inflammation and extracellular matrix (ECM) remodelling. Current therapies for PH, focusing on vasodilatation, do not normalize these activated phenotypes. Furthermore, current approaches to define additional therapeutic targets have focused on determining the initiating signals and their downstream effectors that are important in PH onset and development. Although these approaches have produced a large number of compelling PH treatment targets, many promising human drugs have failed in PH clinical trials. Herein, we propose that one contributing factor to these failures is that processes important in PH development may not be good treatment targets in the established phase of chronic PH. We hypothesize that this is due to alterations of chromatin structure in PH cells, resulting in functional differences between the same factor or pathway in normal or early PH cells versus cells in chronic PH. We propose that the high expression of genes involved in the persistently activated phenotype of PH vascular cells is perpetuated by an open chromatin structure and multiple transcription factors (TFs) via the recruitment of high levels of epigenetic regulators including the histone acetylases P300/CBP, histone acetylation readers including BRDs, the Mediator complex and the positive transcription elongation factor (Abstract figure). Thus, determining how gene expression is controlled by examining chromatin structure, TFs and epigenetic regulators associated with aberrantly expressed genes in pulmonary vascular cells in chronic PH, may uncover new PH therapeutic targets.

Reactive Oxygen Species and Pulmonary Vasculature During Hypobaric Hypoxia

An increasing number of people are living or working at high altitudes (hypobaric hypoxia) and therefore suffering several physiological, biochemical, and molecular changes. Pulmonary vasculature is one of the main and first responses to hypoxia. These responses imply hypoxic pulmonary vasoconstriction (HPV), remodeling, and eventually pulmonary hypertension (PH). These events occur according to the type and extension of the exposure. There is also increasing evidence that these changes in the pulmonary vascular bed could be mainly attributed to a homeostatic imbalance as a result of increased levels of reactive oxygen species (ROS). The increase in ROS production during hypobaric hypoxia has been attributed to an enhanced activity and expression of nicotinamide adenine dinucleotide phosphate oxidase (NADPH oxidase), though there is some dispute about which subunit is involved. This enzymatic complex may be directly induced by hypoxia-inducible factor-1α (HIF-1α). ROS has been found to be related to several pathways, cells, enzymes, and molecules in hypoxic pulmonary vasculature responses, from HPV to inflammation, and structural changes, such as remodeling and, ultimately, PH. Therefore, we performed a comprehensive review of the current evidence on the role of ROS in the development of pulmonary vasculature changes under hypoxic conditions, with a focus on hypobaric hypoxia. This review provides information supporting the role of oxidative stress (mainly ROS) in the pulmonary vasculature’s responses under hypobaric hypoxia and depicting possible future therapeutics or research targets. NADPH oxidase-produced oxidative stress is highlighted as a major source of ROS. Moreover, new molecules, such as asymmetric dimethylarginine, and critical inflammatory cells as fibroblasts, could be also involved. Several controversies remain regarding the role of ROS and the mechanisms involved in hypoxic responses that need to be elucidated.

The MAPK-activator protein-1 signaling regulates changes in lung tissue of rat exposed to hypobaric hypoxia

This study reports the role of MAPKs (JNK, ERK, and p38), and activator protein-1 (AP-1) transcription factor in the hypobaric hypoxia induced change in lung tissue. Healthy male Sprague-Dawley rats were exposed to hypobaric hypoxia for 6, 12, 24, 48, 72, and 120 hr. Hypoxia resulted in significant increase in reactive oxygen species (ROS), vascular endothelial growth factor (VEGF) and decreased nitric oxide (NO), these act as signaling molecules for activation of MAPK and also contribute in development of vascular leakage (an indicator of pulmonary edema) as confirmed by histological studies. Our results confirmed JNK activation as an immediate early response (peaked at 6-48 hr), activation of ERKs (peaked at 24-72 hr) and p38 (peaked at 72-120 hr) as a secondary response to hypoxia. The MAPK pathway up regulated its downstream targets phospho c-Jun (peaked at 6-120 hr), JunB (peaked at 24-120 hr) however, decreased c-Fos, and JunD levels. DNA binding activity also confirmed activation of AP-1 transcription factor in lung tissue under hypobaric hypoxia. Further, we analyzed the proliferative and inflammatory genes regulated by different subunits of AP-1 to explore its role in vascular leakage. Increased expression of cyclin D1 (peaked at 12-72 hr) and p16 level (peaked at 48-120 hr) were correlated to the activation of c-jun, c-Fos and JunB. Administration of NFκB inhibitor caffeic acid phenethyl ester (CAPE) and SP600125 (JNK inhibitor) had no effect on increased levels of Interferon-γ (IFN-γ), Interleukin-1 (IL-1), and Tumor Necrosis Factor-α (TNF-α) thereby confirming the involvement of AP-1 as well as NFκB in inflammation. Expression of c-jun, c-Fos were correlated with activation of proliferative genes and JunB, Fra-1 with pro-inflammatory cytokines. In conclusion immediate response to hypobaric hypoxia induced c-Jun:c-Fos subunits of AP-1; responsible for proliferation that might cause inhomogeneous vasoconstriction leading to vascular leakage and inflammation at increased duration of hypobaric hypoxia exposure.

Novel signaling pathways in pulmonary arterial hypertension (2015 Grover Conference Series)

The proliferative endothelial and smooth muscle cell phenotype, inflammation, and pulmonary vascular remodeling are prominent features of pulmonary arterial hypertension (PAH). Mutations in bone morphogenetic protein type 2 receptor (BMPR2) have been identified as the most common genetic cause of PAH and females with BMPR2 mutations are 2.5 times as likely to develop heritable forms of PAH than males. Higher levels of estrogen have also been observed in males with PAH, implicating sex hormones in PAH pathogenesis. Recently, the estrogen metabolite 16α-OHE1 (hydroxyestrone) was implicated in the regulation of miR29, a microRNA involved in modulating energy metabolism. In females, decreased miR96 enhances serotonin's effect by upregulating the 5-hydroxytryptamine 1B (5HT1B) receptor. Because PAH is characterized as a quasi-malignant disease, likely due to BMPR2 loss of function, altered signaling pathways that sustain this cancer-like phenotype are being explored. Extracellular signal-regulated kinases 1 and 2 and p38 mitogen-activated protein kinases (MAPKs) play a critical role in proliferation and cell motility, and dysregulated MAPK signaling is observed in various experimental models of PAH. Wnt signaling pathways preserve pulmonary vascular homeostasis, and dysregulation of this pathway could contribute to limited vascular regeneration in response to injury. In this review, we take a closer look at sex, sex hormones, and the interplay between sex hormones and microRNA regulation. We also focus on MAPK and Wnt signaling pathways in the emergence of a proproliferative, antiapoptotic endothelial phenotype, which then orchestrates an angioproliferative process of vascular remodeling, with the hope of developing novel therapies that could reverse the phenotype.

Leukotriene B4 Activates Pulmonary Artery Adventitial Fibroblasts in Pulmonary Hypertension

A recent study demonstrated a significant role for leukotriene B4 (LTB4) causing pulmonary vascular remodeling in pulmonary arterial hypertension. LTB4 was found to directly injure luminal endothelial cells and promote growth of the smooth muscle cell layer of pulmonary arterioles. The purpose of this study was to determine the effects of LTB4 on the pulmonary adventitial layer, largely composed of fibroblasts. Here, we demonstrate that LTB4 enhanced human pulmonary artery adventitial fibroblast proliferation, migration, and differentiation in a dose-dependent manner through its cognate G-protein-coupled receptor, BLT1. LTB4 activated human pulmonary artery adventitial fibroblast by upregulating p38 mitogen-activated protein kinase as well as Nox4-signaling pathways. In an autoimmune model of pulmonary hypertension, inhibition of these pathways blocked perivascular inflammation, decreased Nox4 expression, reduced reactive oxygen species production, reversed arteriolar adventitial fibroblast activation, and attenuated pulmonary hypertension development. This study uncovers a novel mechanism by which LTB4 further promotes pulmonary arterial hypertension pathogenesis, beyond its established effects on endothelial and smooth muscle cells, by activating adventitial fibroblasts.

The reversal of pulmonary vascular remodeling through inhibition of p38 MAPK-alpha: a potential novel anti-inflammatory strategy in pulmonary hypertension

The p38 mitogen-activated protein kinase (MAPK) system is increasingly recognized as an important inflammatory pathway in systemic vascular disease but its role in pulmonary vascular disease is unclear. Previous in vitro studies suggest p38 MAPKα is critical in the proliferation of pulmonary artery fibroblasts, an important step in the pathogenesis of pulmonary vascular remodeling (PVremod). In this study the role of the p38 MAPK pathway was investigated in both in vitro and in vivo models of pulmonary hypertension and human disease. Pharmacological inhibition of p38 MAPKα in both chronic hypoxic and monocrotaline rodent models of pulmonary hypertension prevented and reversed the pulmonary hypertensive phenotype. Furthermore, with the use of a novel and clinically available p38 MAPKα antagonist, reversal of pulmonary hypertension was obtained in both experimental models. Increased expression of phosphorylated p38 MAPK and p38 MAPKα was observed in the pulmonary vasculature from patients with idiopathic pulmonary arterial hypertension, suggesting a role for activation of this pathway in the PVremod A reduction of IL-6 levels in serum and lung tissue was found in the drug-treated animals, suggesting a potential mechanism for this reversal in PVremod. This study suggests that the p38 MAPK and the α-isoform plays a pathogenic role in both human disease and rodent models of pulmonary hypertension potentially mediated through IL-6. Selective inhibition of this pathway may provide a novel therapeutic approach that targets both remodeling and inflammatory pathways in pulmonary vascular disease.

Role of platelet-derived growth factor-BB (PDGF-BB) in human pulmonary artery smooth muscle cell proliferation

Pulmonary arterial hypertension (PAH) is a vascular remodeling disease characterized by enhanced proliferation of pulmonary artery smooth muscle cells (PASMCs) and suppressed apoptosis. Platelet-derived growth factor (PDGF) is a potent mitogen involved in cell proliferation and migration. PDGF-BB induces the proliferation and migration of PASMCs and has been proposed to be a key mediator in the progression of PAH. Previous studies have shown that PDGF and its receptor are substantially elevated in lung tissues and PASMCs isolated from patients and animals with PAH, but the underlying mechanisms are still poorly manifested. MAP kinases, including extracellular signal-regulated kinase1/2 (ERK1/2), c-Jun NH2-terminal kinase1/2 (JNK1/2), and p38 are the key intracellular signals for stimuli-induced cell proliferation, survival, and apoptosis. Therefore, the purpose of this study is to determine whether PDGF-BB on cell proliferation process is mediated through the MAP kinases pathway in human PASMCs (HPASMCs). Our results showed PDGF-BB-induced proliferating cell nuclear antigen (PCNA), Cyclin A and Cyclin E expression in a concentration-dependent manner. The expression levels of phosphorylated JNK (p-JNK) was upregulated with 20 ng/ml PDGF-BB treatment, while PDGF-BB could not increase phosphorylated ERK1/2 (p-ERK1/2) and p-38 (p-p38) expression. The effects of PDGF-BB on cell proliferation and survival were weakened after the administration of antagonist of the JNK pathway or si-JNK. In addition, PDGF-BB protected against the loss of mitochondrial membrane potentials evoked by serum deprivation (SD) in a JNK-dependent manner. These results suggest that PDGF-BB promotes HPASMCs proliferation and survival, which is likely to be mediated via the JNK pathway.

Cellular responses to hypoxia in the pulmonary circulation

Hypoxia can be defined as a reduction in available oxygen, whether in a whole organism or in a tissue or cell. It is a real life cause of pulmonary hypertension in humans both in terms of patients with chronic hypoxic lung disease and people living at high altitude. The effect of hypoxia on the pulmonary vasculature can be described in two ways; Hypoxic pulmonary vasoconstriction (HPV) (resulting from smooth muscle cell contraction) and pulmonary vascular remodelling (PVR) (resulting from pulmonary vascular cell proliferation). The pulmonary artery is made up of three resident cell types, the endothelial (intima), smooth muscle (media) and fibroblast (adventitia) cells. This review will examine the effects of hypoxia on the cells of the pulmonary vasculature and give an insight into the possible underlying mechanisms.

Soluble receptor for advanced glycation end products (sRAGE) attenuates haemodynamic changes to chronic hypoxia in the mouse

The calgranulin-like protein MTS1/S100A4 and the receptor for advanced glycation end-products (RAGE) have recently been implicated in mediating pulmonary arterial smooth muscle cell proliferation and vascular remodelling in experimental pulmonary arterial hypertension (PH). Here, the effects of RAGE antagonism upon 2 weeks of hypobaric hypoxia (10% O2)-induced PH in mice were assessed. Treatment with sRAGE was protective against hypobaric hypoxia-induced increases in right ventricular pressure but distal pulmonary vascular remodelling was unaffected. Intralobar pulmonary arteries from hypobaric hypoxic mice treated with sRAGE showed protection against a hypoxia-induced reduction in compliance. However, a combination of sRAGE and hypoxia also dramatically increased the force of contractions to KCl and 5-HT observed in these vessels. The acute addition of sRAGE to the organ bath produced a small, sustained contraction in intralobar pulmonary vessels and produced a synergistic enhancement of the maximal force of contraction in subsequent concentration-response curves to 5-HT. sRAGE had no effect on 5-HT-induced proliferation of Chinese hamster lung fibroblasts (CCL39), used since they have a similar pharmacological profile to mouse pulmonary fibroblasts but, surprisingly, produced a marked increase in hypoxia-induced proliferation. These data implicate RAGE as a modulator of both vasoreactivity and of proliferative processes in the response of the pulmonary circulation to chronic-hypoxia.

The adventitia: Essential role in pulmonary vascular remodeling

A rapidly emerging concept is that the vascular adventitia acts as a biological processing center for the retrieval, integration, storage, and release of key regulators of vessel wall function. It is the most complex compartment of the vessel wall and comprises a variety of cells including fibroblasts, immunomodulatory cells, resident progenitor cells, vasa vasorum endothelial cells, and adrenergic nerves. In response to vascular stress or injury, resident adventitial cells are often the first to be activated and reprogrammed to then influence tone and structure of the vessel wall. Experimental data indicate that the adventitial fibroblast, the most abundant cellular constituent of adventitia, is a critical regulator of vascular wall function. In response to vascular stresses such as overdistension, hypoxia, or infection, the adventitial fibroblast is activated and undergoes phenotypic changes that include proliferation, differentiation, and production of extracellular matrix proteins and adhesion molecules, release of reactive oxygen species, chemokines, cytokines, growth factors, and metalloproteinases that, collectively, affect medial smooth muscle cell tone and growth directly and that stimulate recruitment and retention of circulating inflammatory and progenitor cells to the vessel wall. Resident dendritic cells also participate in "sensing" vascular stress and actively communicate with fibroblasts and progenitor cells to simulate repair processes that involve expansion of the vasa vasorum, which acts as a conduit for further delivery of inflammatory/progenitor cells. This review presents the current evidence demonstrating that the adventitia acts as a key regulator of pulmonary vascular wall function and structure from the "outside in."

MicroRNA-124 controls the proliferative, migratory, and inflammatory phenotype of pulmonary vascular fibroblasts

RATIONALE

Pulmonary hypertensive remodeling is characterized by excessive proliferation, migration, and proinflammatory activation of adventitial fibroblasts. In culture, fibroblasts maintain a similar activated phenotype. The mechanisms responsible for generation/maintenance of this phenotype remain unknown.

OBJECTIVE

We hypothesized that aberrant expression of microRNA-124 (miR-124) regulates this activated fibroblast phenotype and sought to determine the signaling pathways through which miR-124 exerts effects.

METHODS AND RESULTS

We detected significant decreases in miR-124 expression in fibroblasts isolated from calves and humans with severe pulmonary hypertension. Overexpression of miR-124 by mimic transfection significantly attenuated proliferation, migration, and monocyte chemotactic protein-1 expression of hypertensive fibroblasts, whereas anti-miR-124 treatment of control fibroblasts resulted in their increased proliferation, migration, and monocyte chemotactic protein-1 expression. Furthermore, the alternative splicing factor, polypyrimidine tract-binding protein 1, was shown to be a direct target of miR-124 and to be upregulated both in vivo and in vitro in bovine and human pulmonary hypertensive fibroblasts. The effects of miR-124 on fibroblast proliferation were mediated via direct binding to the 3' untranslated region of polypyrimidine tract-binding protein 1 and subsequent regulation of Notch1/phosphatase and tensin homolog/FOXO3/p21Cip1 and p27Kip1 signaling. We showed that miR-124 directly regulates monocyte chemotactic protein-1 expression in pulmonary hypertension/idiopathic pulmonary arterial hypertension fibroblasts. Furthermore, we demonstrated that miR-124 expression is suppressed by histone deacetylases and that treatment of hypertensive fibroblasts with histone deacetylase inhibitors increased miR-124 expression and decreased proliferation and monocyte chemotactic protein-1 production.

CONCLUSIONS

Stable decreases in miR-124 expression contribute to an epigenetically reprogrammed, highly proliferative, migratory, and inflammatory phenotype of hypertensive pulmonary adventitial fibroblasts. Thus, therapies directed at restoring miR-124 function, including histone deacetylase inhibitors, should be investigated.

Inhibition of overactive transforming growth factor-β signaling by prostacyclin analogs in pulmonary arterial hypertension

The heterozygous loss of function mutations in the Type II bone morphogenetic protein receptor (BMPR-II), a member of the transforming growth factor (TGF-β) receptor family, underlies the majority of familial cases of pulmonary arterial hypertension (PAH). The TGF-β1 pathway is activated in PAH, and inhibitors of TGF-β1 signaling prevent the development and progression of PAH in experimental models. However, the effects of currently used therapies on the TGF-β pathway remain unknown. Prostacyclin analogs comprise the first line of treatment for clinical PAH. We hypothesized that these agents effectively decrease the activity of the TGF-β1 pathway. Beraprost sodium (BPS), a prostacyclin analog, selectively inhibits proliferation in a dose-dependent manner in murine primary pulmonary arterial smooth muscle cells (PASMCs) harboring a pathogenic BMPR2 nonsense mutation in both the presence and absence of TGF-β1 stimulation. Our study demonstrates that this agent inhibits TGF-β1-induced SMAD-dependent and SMAD-independent signaling via a protein kinase A-dependent pathway by reducing the phosphorylation of SMADs 2 and 3 and p38 mitogen-activated protein kinase proteins. Finally, in a monocrotaline-induced rat model of PAH, which is associated with increased TGF-β signaling, this study confirms that treprostinil, a stable prostacyclin analog, inhibits the TGF-β pathway by reducing SMAD3 phosphorylation. Taken together, these data suggest that prostacyclin analogs inhibit dysregulated TGF-β signaling in vitro and in vivo, and reduce BMPR-II-mediated proliferation defects in mutant mice PASMCs.

A novel p38 mitogen-activated protein kinase/Elk-1 transcription factor-dependent molecular mechanism underlying abnormal endothelial cell proliferation in plexogenic pulmonary arterial hypertension

Plexiform lesions (PLs), the hallmark of plexogenic pulmonary arterial hypertension (PAH), contain phenotypically altered, proliferative endothelial cells (ECs). The molecular mechanism that contributes to EC proliferation and formation of PLs is poorly understood. We now show that a decrease in intersectin-1s (ITSN-1s) expression due to granzyme B (GrB) cleavage during inflammation associated with PAH and the high p38/Erk1/2(MAPK) activity ratio caused by the GrB/ITSN cleavage products lead to EC proliferation and selection of a proliferative/plexiform EC phenotype. We used human pulmonary artery ECs of PAH subjects (EC(PAH)), paraffin-embedded and frozen human lung tissue, and animal models of PAH in conjunction with microscopy imaging, biochemical, and molecular biology approaches to demonstrate that GrB cleaves ITSN-1s, a prosurvival protein of lung ECs, and generates two biologically active fragments, an N-terminal fragment (GrB-EH(ITSN)) with EC proliferative potential and a C-terminal product with dominant negative effects on Ras/Erk1/2. The proliferative potential of GrB-EH(ITSN) is mediated via sustained phosphorylation of p38(MAPK) and Elk-1 transcription factor and abolished by chemical inhibition of p38(MAPK). Moreover, lung tissue of PAH animal models and human specimens and EC(PAH) express lower levels of ITSN-1s compared with controls and the GrB-EH(ITSN) cleavage product. Moreover, GrB immunoreactivity is associated with PLs in PAH lungs. The concurrent expression of the two cleavage products results in a high p38/Erk1/2(MAPK) activity ratio, which is critical for EC proliferation. Our findings identify a novel GrB-EH(ITSN)-dependent pathogenic p38(MAPK)/Elk-1 signaling pathway involved in the poorly understood process of PL formation in severe PAH.

Extract from Mimosa pigra attenuates chronic experimental pulmonary hypertension

ETHNOPHARMACOLOGICAL RELEVANCE

Different parts of Mimosa pigra (MPG) are used in traditional medicine in Madagascar, tropical Africa, South America and Indonesia for various troubles including cardiovascular disorders.

AIM OF THE STUDY

To investigate the mechanisms underlying the vascular effects of MPG by assessing in vitro its antioxidant and anti-inflammatory properties, and its vascular relaxing effects, and in vivo, its action on hypoxic pulmonary hypertension (PAH) in rats.

MATERIAL AND METHODS

The antioxidant activity of MPG leaf hydromethanolic extract was determined by using both the 1,1-diphenyl-2-picrylhydrazyl radical scavenging and the oxygen radical absorbance capacity in vitro assays. Anti-inflammatory properties were assayed on TNFα-induced VCAM-1 expression in endothelial cells. The vasorelaxant effect of MPG extract was studied on rat arterial rings pre-contracted with phenylephrine (1μM) in the presence or absence of the endothelium. In vivo MPG extract effects were analyzed in chronic hypoxic PAH, obtained by housing male Wistar rats, orally treated or not with MPG extract (400mg/kg/d), in a hypobaric chamber for 21 days.

RESULTS

MPG leaf extract had antioxidant and anti-inflammatory properties. It induced endothelium-dependent, NO-mediated relaxation of rat aorta and pulmonary artery. In vivo, chronic MPG treatment reduced hypoxic PAH in rat by decreasing by 22.3% the pulmonary arterial pressure and by 20.0% and 23.9% the pulmonary artery and cardiac remodelling, respectively. This effect was associated with a restoration of endothelium function and a 2.3-fold increase in endothelial NO synthase phosphorylation. MPG leaf hydromethanolic extract contained tryptophan and flavonoids, including quercetin glycosides. Both compounds also efficiently limit hypoxia-induced PAH.

CONCLUSIONS

Our results show endothelial protective action of MPG leaf hydromethanolic extract which is likely to be due to its antioxidant action. MPG successfully attenuated the development of PAH, thus demonstrating the protective effect of MPG on cardiovascular diseases.

Activity of the estrogen-metabolizing enzyme cytochrome P450 1B1 influences the development of pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH) is a hyperproliferative vascular disorder observed predominantly in women. Estrogen is a potent mitogen in human pulmonary artery smooth muscle cells and contributes to PAH in vivo; however, the mechanisms attributed to this causation remain obscure. Curiously, heightened expression of the estrogen-metabolizing enzyme cytochrome P450 1B1 (CYP1B1) is reported in idiopathic PAH and murine models of PAH.

METHODS AND RESULTS

Here, we investigated the putative pathogenic role of CYP1B1 in PAH. Quantitative reverse transcription-polymerase chain reaction, immunoblotting, and in situ analysis revealed that pulmonary CYP1B1 is increased in hypoxic PAH, hypoxic+SU5416 PAH, and human PAH and is highly expressed within the pulmonary vascular wall. PAH was assessed in mice via measurement of right ventricular hypertrophy, pulmonary vascular remodeling, and right ventricular systolic pressure. Hypoxic PAH was attenuated in CYP1B1(-/-) mice, and the potent CYP1B1 inhibitor 2,3',4,5'-tetramethoxystilbene (TMS; 3 mg · kg(-1) · d(-1) IP) significantly attenuated hypoxic PAH and hypoxic+SU5416 PAH in vivo. TMS also abolished estrogen-induced proliferation in human pulmonary artery smooth muscle cells and PAH-pulmonary artery smooth muscle cells. The estrogen metabolite 16α-hydroxyestrone provoked human pulmonary artery smooth muscle cell proliferation, and this mitogenic effect was greatly pronounced in PAH-pulmonary artery smooth muscle cells. ELISA analysis revealed that 16α-hydroxyestrone concentration was elevated in PAH, consistent with CYP1B1 overexpression and activity. Finally, administration of the CYP1B1 metabolite 16α-hydroxyestrone (1.5 mg · kg(-1) · d(-1) IP) caused the development of PAH in mice.

CONCLUSIONS

Increased CYP1B1-mediated estrogen metabolism promotes the development of PAH, likely via the formation of mitogens, including 16α-hydroxyestrone. Collectively, this study reveals a possible novel therapeutic target in clinical PAH.

Hypoxia induces unique proliferative response in adventitial fibroblasts by activating PDGFβ receptor-JNK1 signalling

AIMS

Pulmonary hypertension (PH) is a devastating condition for which no disease-modifying therapies exist. PH is recognized as proliferative disease of the pulmonary artery (PA). In the experimental newborn calf model of hypoxia-induced PH, adventitial fibroblasts in the PA wall exhibit a heightened replication index. Because elevated platelet-derived growth factor β receptor (PDGFβ-R) signalling is associated with PH, we tested the hypothesis that the activation of PDGFβ-R contributes to fibroblast proliferation and adventitial remodelling in PH.

METHODS AND RESULTS

Newborn calves were exposed to either ambient air (P(B) = 640 mmHg) (Neo-C) or high altitude (P(B) = 445 mm Hg) (Neo-PH) for 2 weeks. PDGFβ-R phosphorylation was markedly elevated in PA adventitia of Neo-PH calves as well as in cultured PA fibroblasts isolated from Neo-PH animals. PDGFβ-R activation with PDGF-BB stimulated higher replication in Neo-PH cells compared with that of control fibroblasts. PDGF-BB-induced proliferation was dependent on reactive oxygen species generation and extracellular signal-regulated kinase1/2 activation in both cell populations; however, only Neo-PH cell division via PDGFβ-R activation displayed a unique dependence on c-Jun N-terminal kinase1 (JNK1) stimulation as the blockade of JNK1 with SP600125, a pharmacological antagonist of the JNK pathway, and JNK1-targeted siRNA selectively blunted Neo-PH cell proliferation.

CONCLUSIONS

Our data strongly suggest that hypoxia-induced modified cells engage the PDGFβ-R-JNK1 axis to confer distinctively heightened proliferation and adventitial remodelling in PH.

Osteopontin is an endogenous modulator of the constitutively activated phenotype of pulmonary adventitial fibroblasts in hypoxic pulmonary hypertension

Increased cell proliferation and migration, of several cell types are key components of vascular remodeling observed in pulmonary hypertension (PH). Our previous data demonstrate that adventitial fibroblasts isolated from pulmonary arteries of chronically hypoxic hypertensive calves (termed PH-Fibs) exhibit a "constitutively activated" phenotype characterized by high proliferative and migratory potential. Osteopontin (OPN) has been shown to promote several cellular activities including growth and migration in cancer cells. We thus tested the hypothesis that elevated OPN expression confers the "activated" highly proproliferative and promigratory/invasive phenotype of PH-Fibs. Our results demonstrate that, both in vivo and ex vivo, PH-Fibs exhibited increased expression of OPN, as well as its cognate receptors, α(V)β(3) and CD44, compared with control fibroblasts (CO-Fibs). Augmented OPN expression in PH-Fibs corresponded to their high proliferative, migratory, and invasive properties and constitutive activation of ERK1/2 and AKT signaling. OPN silencing via small interfering RNA or sequestering OPN production by specific antibodies led to decreased proliferation, migration, invasion, and attenuated ERK1/2, AKT phosphorylation in PH-Fibs. Furthermore, increasing OPN levels in CO-Fibs via recombinant OPN resulted in significant increases in their proliferative, migratory, and invasive capabilities to the levels resembling those of PH-Fibs. Thus our data suggest OPN as an essential contributor to the activated (highly proliferative, migratory, and proinvasive) phenotype of pulmonary adventitial fibroblasts in hypoxic PH.

BMPR-II deficiency elicits pro-proliferative and anti-apoptotic responses through the activation of TGFβ-TAK1-MAPK pathways in PAH

Pulmonary arterial hypertension (PAH) is a cardiovascular disorder associated with enhanced proliferation and suppressed apoptosis of pulmonary arterial smooth muscle cells (PASMCs). Heterozygous mutations in the type II receptor for bone morphogenetic protein (BMPR2) underlie the majority of the inherited and familial forms of PAH. The transforming growth factor β (TGFβ) pathway is activated in both human and experimental models of PAH. However, how these factors exert pro-proliferative and anti-apoptotic responses in PAH remains unclear. Using mouse primary PASMCs derived from knock-in mice, we demonstrated that BMPR-II dysfunction promotes the activation of small mothers against decapentaplegia-independent mitogen-activated protein kinase (MAPK) pathways via TGFβ-associated kinase 1 (TAK1), resulting in a pro-proliferative and anti-apoptotic response. Inhibition of the TAK1-MAPK axis rescues abnormal proliferation and apoptosis in these cells. In both hypoxia and monocrotaline-induced PAH rat models, which display reduced levels of bmpr2 transcripts, this study further indicates that the TGFβ-MAPK axis is activated in lungs following elevation of both expression and phosphorylation of the TAK1 protein. In ex vivo cell-based assays, TAK1 inhibits BMP-responsive reporter activity and interacts with BMPR-II receptor. In the presence of pathogenic BMPR2 mutations observed in PAH patients, this interaction is greatly reduced. Taken together, these data suggest dysfunctional BMPR-II responsiveness intensifies TGFβ-TAK1-MAPK signalling and thus alters the ratio of apoptosis to proliferation. This axis may be a potential therapeutic target in PAH.

Low-dose fluvastatin reverses the hypoxic pulmonary adventitial fibroblast phenotype in experimental pulmonary hypertension

Hypoxic pulmonary hypertension is a worldwide public health problem. Statins attenuate hypoxic pulmonary hypertension in animal models, but the mechanism of action and applicability of these results to human treatment are not established. In hypoxic models, pulmonary artery fibroblast proliferation contributes substantially to pulmonary vascular remodeling. We previously showed that acute hypoxic pulmonary adventitial fibroblast proliferation can be selectively inhibited by statins and p38 mitogen-activated protein (MAP) kinase inhibitors. Here we used complementary chronic hypoxic and acute hypoxic coculture models to obtain necessary preclinical information regarding the utility of fluvastatin in the treatment of chronic hypoxic pulmonary hypertension. The effects of fluvastatin, cholesterol pathway intermediates, and related inhibitors on hypoxic adventitial fibroblast proliferation, p38 MAP kinase phosphorylation, and pulmonary artery smooth muscle cell proliferation were determined, using complementary chronic hypoxic rat and acute hypoxic bovine cell models. Fluvastatin reversed the proliferative phenotypic switch in adventitial fibroblasts from chronic hypoxic animals. This effect was circulation-specific, and implicated a Rac1-p38 MAP kinase signaling pathway. Coculture and conditioned media experiments also implicated this statin-sensitive signaling pathway in the release of pulmonary artery smooth muscle cell mitogens by hypoxic pulmonary adventitial fibroblasts. Treprostinil, sildenafil, and bosentan exerted no effect on the hypoxic fibroblast phenotype. Phenotypic changes (increased proliferation and mitogen release) in pulmonary artery fibroblasts during chronic hypoxia are dependent on a Rac1-p38 MAP kinase signaling pathway. The inhibition of these phenotypic changes with fluvastatin may be therapeutically relevant in high-altitude residents and in patients with hypoxic lung disease.

Schistosomiasis-induced experimental pulmonary hypertension: role of interleukin-13 signaling

The mechanisms underlying schistosomiasis-induced pulmonary hypertension (PH), one of the most common causes of PH worldwide, remain unclear. We sought to determine whether Schistosoma mansoni causes experimental PH associated with pulmonary vascular remodeling in an interleukin (IL)-13-dependent manner. IL-13Ralpha1 is the canonical IL-13 signaling receptor, whereas IL-13Ralpha2 is a competitive nonsignaling decoy receptor. Wild-type, IL-13Ralpha1(-/-), and IL-13Ralpha2(-/-) C57BL/6J mice were percutaneously infected with S. mansoni cercariae, followed by i.v. injection of eggs. We assessed PH with right ventricular catheterization, histological evaluation of pulmonary vascular remodeling, and detection of IL-13 and transforming growth factor-beta signaling. Infected mice developed pulmonary peri-egg granulomas and arterial remodeling involving predominantly the vascular media. In addition, gain-of-function IL-13Ralpha2(-/-) mice had exacerbated vascular remodeling and PH. Mice with loss of IL-13Ralpha1 function did not develop PH and had reduced pulmonary vascular remodeling. Moreover, the expression of resistin-like molecule-alpha, a target of IL-13 signaling, was increased in infected wild-type and IL-13Ralpha2(-/-) but not IL-13Ralpha1(-/-) mice. Phosphorylated Smad2/3, a target of transforming growth factor-beta signaling, was increased in both infected mice and humans with the disease. Our data indicate that experimental schistosomiasis causes PH and potentially relies on up-regulated IL-13 signaling.

Protective role of the antidiabetic drug metformin against chronic experimental pulmonary hypertension

BACKGROUND AND PURPOSE

Pulmonary arterial hypertension (PAH) is associated with increased contraction and proliferation of pulmonary vascular smooth muscle cells. The anti-diabetic drug metformin has been shown to have relaxant and anti-proliferation properties. We thus examined the effect of metformin in PAH.

EXPERIMENTAL APPROACH

Metformin effects were analysed in hypoxia- and monocrotaline-induced PAH in rats. Ex vivo and in vitro analyses were performed in lungs, pulmonary artery rings and cells.

KEY RESULTS

In hypoxia- and monocrotaline-induced PAH, the changes in mean pulmonary arterial pressure and right heart hypertrophy were nearly normalized by metformin treatment (100 mg.kg(-1).day(-1)). Pulmonary arterial remodelling occurring in both experimental models of PAH was also inhibited by metformin treatment. In rats with monocrotaline-induced PAH, treatment with metformin significantly increased survival. Metformin increased endothelial nitric oxide synthase phosphorylation and decreased Rho kinase activity in pulmonary artery from rats with PAH. These effects are associated with an improvement of carbachol-induced relaxation and reduction of phenylephrine-induced contraction of pulmonary artery. In addition, metformin inhibited mitogen-activated protein kinase activation and strongly reduced pulmonary arterial cell proliferation during PAH. In vitro, metformin directly inhibited pulmonary artery smooth muscle cell growth.

CONCLUSIONS AND IMPLICATIONS

Metformin protected against PAH, regardless of the initiating stimulus. This protective effect may be related to its anti-remodelling property involving improvement of endothelial function, vasodilatory and anti-proliferative actions. As metformin is currently prescribed to treat diabetic patients, assessment of its use as a therapy against PAH in humans should be easier.

Chronic hypoxia increases pressure-dependent myogenic tone of the uterine artery in pregnant sheep: role of ERK/PKC pathway

Chronic hypoxia during pregnancy has profound effects on uterine artery (UA) contractility and attenuates uterine blood flow. The present study tested the hypothesis that chronic hypoxia inhibits the pregnancy-induced reduction in pressure-dependent myogenic tone of resistance-sized UAs. UAs were isolated from nonpregnant ewes (NPUAs) and near-term pregnant ewes (PUAs) that had been maintained at sea level (approximately 300 m) or at high altitude (3,801 m) for 110 days. In normoxic animals, the pressure-dependent myogenic response was significantly attenuated in PUAs compared with NPUAs. Hypoxia significantly increased myogenic tone in PUAs and abolished its difference between PUAs and NPUAs. Consistently, there was a significant increase in PKC-mediated baseline Ca(2+) sensitivity of PUAs in hypoxic animals. Hypoxia significantly increased phorbol 12,13-dibutyrate (PDBu)-induced contractions in PUAs but not in NPUAs. Whereas the inhibition of ERK1/2 by PD-98059 potentiated PDBu-mediated contractions of PUAs in normoxic animals, it failed to do so in hypoxic animals. Hypoxia decreased ERK1/2 expression in PUAs. PDBu induced membrane translocation of PKC-alpha and PKC-epsilon. Whereas there were no significant differences in PKC-alpha translocation among all groups, the translocation of PKC-epsilon was significantly enhanced in NPUAs compared with PUAs in normoxic animals, and hypoxia significantly increased PKC-epsilon translocation in PUAs. In the presence of PD-98059, there were no significant differences in PDBu-induced PKC-epsilon translocation among all groups. Treatment of PUAs isolated from normoxic animals with 10.5% O(2) for 48 h ex vivo significantly increased PDBu-induced contractions and eliminated its difference between PUAs and NPUAs. The results suggest that hypoxia upregulates pressure-dependent myogenic tone through its direct effect in suppressing ERK1/2 activity and increasing the PKC signal pathway, leading to an increase in the Ca(2+) sensitivity of the myogenic mechanism in the UA during pregnancy.

Inhibition of p38 MAPK reverses hypoxia-induced pulmonary artery endothelial dysfunction

Hypoxia-induced endothelial dysfunction plays a crucial role in the pathogenesis of hypoxic pulmonary hypertension. p38 MAPK expression is increased in the pulmonary artery following hypoxic exposure. Recent evidence suggests that increased p38 MAPK activity is associated with endothelial dysfunction. However, the role of p38 MAPK activation in pulmonary artery endothelial dysfunction is not known. Sprague-Dawley rats were exposed to 2 wk hypobaric hypoxia, which resulted in the development of pulmonary hypertension and vascular remodeling. Endothelium-dependent relaxation of intrapulmonary vessels from hypoxic animals was impaired due to a reduced nitric oxide (NO) generation. This was despite increased endothelial NO synthase immunostaining and protein expression. Hypoxia exposure increased superoxide generation and p38 MAPK expression. The inhibition of p38 MAPK restored endothelium-dependent relaxation, increased bioavailable NO, and reduced superoxide production. In conclusion, the pharmacological inhibition of p38 MAPK was effective in increasing NO generation, reducing superoxide burden, and restoring hypoxia-induced endothelial dysfunction in rats with hypoxia-induced pulmonary hypertension. p38 MAPK may be a novel target for the treatment of pulmonary hypertension.

Pulmonary hypertension: therapeutic targets within the serotonin system

Pulmonary arterial hypertension (PAH) is characterized by a sustained and progressive elevation in pulmonary arterial pressure and pulmonary vascular remodelling leading to right heart failure and death. Prognosis is poor and novel therapeutic approaches are needed. The serotonin hypothesis of PAH originated in the 1960s after an outbreak of the disease was reported among patients taking the anorexigenic drugs aminorex and fenfluramine. These are indirect serotonergic agonists and serotonin transporter substrates. Since then many advances have been made in our understanding of the role of serotonin in the pathobiology of PAH. The rate-limiting enzyme in the synthesis of serotonin is tryptophan hydroxylase (Tph). Serotonin is synthesized, through Tph1, in the endothelial cells of the pulmonary artery and can then act on underlying pulmonary arterial smooth muscle cells and pulmonary arterial fibroblasts in a paracrine fashion causing constriction and remodelling. These effects of serotonin can be mediated through both the serotonin transporter and serotonin receptors. This review will discuss our current understanding of 'the serotonin hypothesis' of PAH and highlight possible therapeutic targets within the serotonin system.

Genomic assessment of a multikinase inhibitor, sorafenib, in a rodent model of pulmonary hypertension

Pulmonary hypertension (PH) and cancer pathology share growth factor- and MAPK stress-mediated signaling pathways resulting in endothelial and smooth muscle cell dysfunction and angioproliferative vasculopathy. In this study, we assessed sorafenib, an antineoplastic agent and inhibitor of multiple kinases important in angiogenesis [VEGF receptor (VEGFR)-1-3, PDGF receptor (PDGFR)-beta, Raf-1 kinase] as a potential PH therapy. Two PH rat models were used: a conventional hypoxia-induced PH model and an augmented PH model combining dual VEGFR-1 and -2 inhibition (SU-5416, single 20 mg/kg injection) with hypoxia. In addition to normoxia-exposed control animals, four groups were maintained at 10% inspired O(2) fraction for 3.5 wk (hypoxia/vehicle, hypoxia/SU-5416, hypoxia/sorafenib, and hypoxia/SU-5416/sorafenib). Compared with normoxic control animals, rats exposed to hypoxia/SU-5416 developed hemodynamic and histological evidence of severe PH while rats exposed to hypoxia alone displayed only mild elevations in hemodynamic values (pulmonary vascular and right ventricular pressures). Sorafenib treatment (daily gavage, 2.5 mg/kg) prevented hemodynamic changes and demonstrated dramatic attenuation of PH-associated vascular remodeling. Compared with normoxic control rats, expression profiling (Affymetrix platform) of lung RNA obtained from hypoxia [false discovery rate (FDR) 6.5%]- and hypoxia/SU-5416 (FDR 1.6%)-challenged rats yielded 1,019 and 465 differentially regulated genes (fold change >1.4), respectively. A novel molecular signature consisting of 38 differentially expressed genes between hypoxia/SU-5416 and hypoxia/SU-5416/sorafenib (FDR 6.7%) was validated by either real-time RT-PCR or immunoblotting. Finally, immunoblotting studies confirmed the upregulation of the MAPK cascade in both PH models, which was abolished by sorafenib. In summary, sorafenib represents a novel potential treatment for severe PH with the MAPK cascade a potential canonical target.

Smooth muscle alpha-actin expression and myofibroblast differentiation by TGFbeta are dependent upon MK2

Fibroblasts play a major role in processes such as wound repair, scarring, and fibrosis. Differentiation into myofibroblasts, characterized by upregulation of smooth muscle alpha-actin (smalpha) in response to profibrotic agents such as TGFbeta is believed to be an important step in fibrosis. Therefore, elucidating mechanisms of myofibroblast differentiation might reveal novel targets in treating diseases such as idiopathic pulmonary fibrosis (IPF). MK2 is a kinase substrate of p38 MAP kinase that mediates some effects of p38 activation on the actin cytoskeleton. Using mouse embryonic fibroblasts (MEF) from MK2 knockout (MK2(-/-)) mice, we demonstrate that disrupting expression of MK2 expression reduces filamentous actin and stress fibers. It also causes MK2(-/-) MEF to express less smalpha than their corresponding wild-type (WT) MEF at baseline and in response to TGFbeta. Furthermore, TGFbeta causes downregulation of smalpha in MK2(-/-) MEF, instead of upregulation observed in WT MEF. Expression of other fibroblast markers, such as collagen, is not altered in MK2(-/-) MEF. Our results further suggest that downregulation of smalpha in MK2(-/-) MEF is not due to lack of activation of serum responsive promoter elements, but probably due to reduced smalpha message stability in these cells. These results indicate that MK2 plays a key role in regulation of smalpha expression, and that targeting MK2 might present a therapeutic approach in managing conditions such as pulmonary fibrosis.

Fluvastatin inhibits hypoxic proliferation and p38 MAPK activity in pulmonary artery fibroblasts

The earliest structural change in hypoxia-induced pulmonary hypertension is increased proliferation of adventitial fibroblasts. This fibroproliferative response occurs in acute and chronic hypoxic models, is dependent on p38 mitogen-activated protein (MAP) kinase activation, is selective for the pulmonary circulation, and would seem an important therapeutic target. Simvastatin attenuates pulmonary vascular remodeling in animal models, but additional information regarding mechanisms of action, differential antiproliferative effects and dose responses of available statins is required for appropriate clinical trial design. Our objectives were to determine the effects of statins on acute hypoxia-induced proliferation and p38 MAP kinase activation in pulmonary and systemic artery fibroblasts, to assess the effects of cholesterol intermediates, prenyltransferase and related inhibitors, and to determine the statin's mechanism of action. Atorvastatin, fluvastatin, and simvastatin inhibited adventitial fibroblast proliferation. At low doses (1 microM), this effect was selective for hypoxic (versus serum-induced) proliferation and was also selective for pulmonary (versus systemic) fibroblasts. Complete inhibition of hypoxia-induced p38 MAP kinase activity was achieved at this 1-microM dose. The lipophilic statins exhibited similar potency. The statin effect was reversed by geranylgeranyl pyrophosphate and mimicked by geranylgeranyl transferase and Rac1 inhibitors. Hypoxia-induced p38 MAP kinase activation and proliferation in pulmonary adventitial fibroblasts is dependent on a geranylgeranylated signaling protein, probably Rac1. One micromolar of fluvastatin exhibits a circulation- and stimulus-selective antiproliferative effect on pulmonary artery fibroblasts. The pharmacokinetics of fluvastatin would suggest that its antiproliferative effects may be useful in pulmonary hypertension associated with hypoxia.

Modèles animaux d’hypertension artérielle pulmonaire

INTRODUCTION

Pulmonary arterial hypertension (PAH) is a rare syndrome of fatigue and dyspnoea, caused by increased pulmonary vascular resistance and right heart failure without an identifiable pulmonary or cardiac cause. Despite important recent advances in treatment the condition remains incurable.

BACKGROUND

Experimental animal models of PAH rely on hypoxic or monocrotaline injected rodents, the creation of left to right shunts in lambs or piglets, ligation of the ductus arteriosus in newborn lambs, genetically manipulated rodents and tissue culture. Hypoxic pulmonary hypertension is usually only moderate and limited to medial hypertrophy with varying degrees of adventitial change, but may progress to extensive remodelling in some species. Monocrotaline induced pulmonary hypertension is severe with prominent medial hypertrophy, inflammatory adventitial remodelling and, initially, pulmonary oedema and endothelial apoptosis. Pulmonary hypertension induced by shunting remains the most realistic model of PAH but causes only moderate increase in vascular resistance due to medial hypertrophy. Pulmonary hypertension of the newborn is severe but largely vasospastic, with predominant medial hypertrophy. An increasing number of genetically manipulated rodents are becoming available for the investigation of specific signalling pathways.

VIEWPOINT

While none of the models has yet reproduced PAH each allows investigation of a specific hypothesis. Recent progress has resulted from genetic manipulation and molecular and cellular approaches.

CONCLUSIONS

Animal models of PAH share basic biological abnormalities which, together with the study of lung tissue from patients with severe disease should lead to better understanding of the pathology and therapeutic innovation.

Hypoxia-induced pulmonary vascular remodeling: cellular and molecular mechanisms

Chronic hypoxic exposure induces changes in the structure of pulmonary arteries, as well as in the biochemical and functional phenotypes of each of the vascular cell types, from the hilum of the lung to the most peripheral vessels in the alveolar wall. The magnitude and the specific profile of the changes depend on the species, sex, and the developmental stage at which the exposure to hypoxia occurred. Further, hypoxia-induced changes are site specific, such that the remodeling process in the large vessels differs from that in the smallest vessels. The cellular and molecular mechanisms vary and depend on the cellular composition of vessels at particular sites along the longitudinal axis of the pulmonary vasculature, as well as on local environmental factors. Each of the resident vascular cell types (ie, endothelial, smooth muscle, adventitial fibroblast) undergo site- and time-dependent alterations in proliferation, matrix protein production, expression of growth factors, cytokines, and receptors, and each resident cell type plays a specific role in the overall remodeling response. In addition, hypoxic exposure induces an inflammatory response within the vessel wall, and the recruited circulating progenitor cells contribute significantly to the structural remodeling and persistent vasoconstriction of the pulmonary circulation. The possibility exists that the lung or lung vessels also contain resident progenitor cells that participate in the remodeling process. Thus the hypoxia-induced remodeling of the pulmonary circulation is a highly complex process where numerous interactive events must be taken into account as we search for newer, more effective therapeutic interventions. This review provides perspectives on each of the aforementioned areas.

p38 MAP kinase: essential role in hypoxia-mediated human pulmonary artery fibroblast proliferation

Pulmonary arterial hypertension (PAH) is a disease that results in thickening of the vascular wall. Some of the most prominent changes are seen in the adventitia as a result of fibroblast proliferation and increased extracellular matrix deposition. Previous work from this laboratory using animal models has shown that pulmonary but not systemic artery fibroblasts proliferate to hypoxic exposure and that this response is dependent on activation of p38 mitogen-activated protein kinase (p38MAPK). In this study, we wished to determine whether human pulmonary artery fibroblasts (HPAFs) behaved similarly under conditions of acute hypoxic exposure (35 mmHg for 24 h). Fibroblast proliferation was assessed by [(3)H]thymidine uptake and protein assays performed using Western blotting techniques. HPAFs proliferated in response to acute hypoxic exposure, human systemic artery fibroblasts did not. This hypoxia-mediated proliferation was p38 MAPK dependent and could be blocked using a specific p38 MAPK inhibitor. Hypoxia-inducible factor-1 (HIF-1) expression was increased in hypoxic pulmonary but not systemic cells and could be partially abrogated with the p38 inhibitor. This work in man confirmed our previous findings in animals that significant differences exist between the pulmonary and systemic circulations in response to hypoxic exposure. This study highlights the importance of p38 MAPK and HIF-1 in hypoxia-mediated proliferation of pulmonary artery adventitial fibroblasts.

Role of the adventitia in pulmonary vascular remodeling

An increasing volume of experimental data indicates that the adventitial fibroblast, in both the pulmonary and systemic circulations, is a critical regulator of vascular wall function in health and disease. A rapidly emerging concept is that the vascular adventitia acts as biological processing center for the retrieval, integration, storage, and release of key regulators of vessel wall function. In response to stress or injury, resident adventitial cells can be activated and reprogrammed to exhibit different functional and structural behaviors. In fact, under certain conditions, the adventitial compartment may be considered the principal injury-sensing tissue of the vessel wall. In response to vascular stresses such as overdistension and hypoxia, the adventitial fibroblast is activated and undergoes phenotypic changes, which include proliferation, differentiation, upregulation of contractile and extracellular matrix proteins, and release of factors that directly affect medial smooth muscle cell tone and growth and that stimulate recruitment of inflammatory and progenitor cells to the vessel wall. Each of these changes in fibroblast phenotype modulates either directly or indirectly changes in overall vascular function and structure. The purpose of this review is to present the current evidence demonstrating that the adventitial fibroblast acts as a key regulator of pulmonary vascular function and structure from the "outside-in."

Effect of propofol and etomidate on normoxic and chronically hypoxic pulmonary artery

Background

Chronic alveolar hypoxia results in sustained arterial constriction, and increase in pulmonary vascular resistance leading to pulmonary artery hypertension (PAHT). The aim of this study was to investigate the effect of propofol and etomidate on pulmonary artery (PA) reactivity in chronically hypoxic (CH) rats, a model of pulmonary arterial hypertension (PAHT), in normoxic animals, and human PA.

Methods

CH rats were maintained 14 days at 380 mmHg pressure in a hypobaric chamber. Human tissue was retrieved from histological lung pieces from patients undergoing resection for carcinoma. Cumulative concentrations of anaesthetics were tested on isolated vascular rings precontracted with phenylephrine (PHE) or 100 mM KCl. Statistical comparisons were done by ANOVA, followed, when needed, by Student t tests with Bonferroni correction as post-hoc tests.

Results

In normoxic rat PA, maximal relaxation (R_max) induced by etomidate and propofol was 101.3 ± 0.8% and 94.0 ± 2.3%, respectively, in KCl-precontracted rings, and 63.3 ± 9.7% and 46.1 ± 9.1%, respectively, in PHE-precontracted rings (n = 7). In KCl-precontracted human PA, R_max was 84.7 ± 8.6 % and 66.5 ± 11.8%, for etomidate and propofol, respectively, and 154.2 ± 22.4 % and 51.6 ± 15.1 %, respectively, in PHE-precontracted human PA (n = 7). In CH rat PA, the relaxant effect of both anaesthetics was increased in PHE-precontracted and, for etomidate only, in KCl-precontracted PA. In aorta, CH induced no change in the relaxant effect of anaesthetics.

Conclusion

Propofol and etomidate have relaxant properties in PA from human and normoxic rat. The relaxant effect is specifically accentuated in PA from CH rat, mainly via an effect on the pharmacomechanical coupling. Etomidate appears to be more efficient than propofol at identical concentration, but, taking into account clinical concentrations, etomidate is less potent than propofol, which effect was in the range of clinical doses. Although these findings provide experimental support for the preferential use of etomidate for haemodynamic stability in patients suffering from PAHT, the clinical relevance of the observations requires further investigation.

Downregulation of type II bone morphogenetic protein receptor in hypoxic pulmonary hypertension

Heterozygous mutations in the type II receptor for bone morphogenetic protein (BMPR-II) and dysfunction of BMPR-II have been implicated in patients with primary pulmonary hypertension (PH). To clarify the possible involvement of BMP and BMPR-II in the development of hypoxic PH, the expression of BMP-2, BMPR-II, and their downstream signals were investigated in rat lung under normal and hypoxic conditions by RT-PCR, immunoblot, and immunohistochemical methods. In rats under normal conditions, BMP-2 is localized in the endothelium of the pulmonary artery, whereas BMPR-II is abundantly expressed in the endothelium, smooth muscle cells, and adventitial fibroblasts. After 0.5 and 3 days of exposure to hypoxia, upregulation of BMP-2 was observed in the intrapulmonary arteries. The change was accompanied by activation of its downstream signaling, p38 MAPK, and Erk1/2 MAPK, and the apoptotic process, measured by caspase-3 activity and TdT-mediated dUTP nick end labeling-positive cells. In contrast, a significant decrease in the expression of BMPR-II and inactivation of p38 MAPK and caspase-3 were observed in the pulmonary vasculature after 7–21 days of hypoxia exposure. Because BMP-2 is known to inhibit proliferation of vascular smooth muscle cells and promote cellular apoptosis, disruption of BMP signaling pathway through downregulation of BMPR-II in chronic hypoxia may result in pulmonary vascular remodeling due to the failure of critical antiproliferative/differentiation programs in the pulmonary vasculature. These results suggest abrogation of BMP signaling may be a common molecular pathogenesis in the development of PH with various pathophysiological events, including primary and hypoxic PH.

Impact of HIF‐1α and HIF‐2α on proliferation and migration of human pulmonary artery fibroblasts in hypoxia

Proliferation of adventitial fibroblasts of small intrapulmonary arteries (FBPA) has been disclosed as an early event in the development of pulmonary hypertension and cor pulmonale in response to hypoxia. We investigated the role of hypoxia-inducible transcription factors (HIF) in human FBPA exposed to hypoxia. Primary cultures of FBPA displayed a strong mitogenic response to 24 h hypoxia, whereas the rate of apoptosis was significantly suppressed. In addition, the migration of FBPA was strongly increased under hypoxic conditions but not the expression of alpha-smooth muscle actin. Hypoxia induced a marked up-regulation (protein level) of both HIF-1alpha and HIF-2alpha, alongside with nuclear translocation of these transcription factors. Specific inhibition of either HIF-1alpha or HIF-2alpha was achieved by RNA interference technology, as proven by HIF-1alpha and HIF-2alpha mRNA and protein analysis and expression analysis of HIF downstream target genes. With the use of this approach, the hypoxia-induced proliferative response of the FBPA was found to be solely HIF-2alpha dependent, whereas the migratory response was significantly reduced by both HIF-1alpha and HIF-2alpha interference. In conclusion, HIF up-regulation is essential for hypoxic cellular responses in human pulmonary artery adventitial fibroblasts such as proliferation and migration, mimicking the pulmonary hypertensive phenotype in vivo. Differential HIF subtype dependency was noted, with HIF-2alpha playing a predominant role, which may offer future intervention strategies.

p38 MAP kinase isoform activity and cell cycle regulators in the proliferative response of pulmonary and systemic artery fibroblasts to acute hypoxia

Many cardiopulmonary diseases are associated with pulmonary hypertension which adds significant co-morbidity. Pulmonary hypertension is due partly to vasoconstriction but sustained by pulmonary vascular remodelling. If pathological endpoints are to be reversed in patients with pulmonary hypertension, the processes by which vascular remodelling occur need to be determined. Hypoxia provides a good model of pulmonary hypertension. We have previously shown that chronic hypoxia results in increased proliferation of pulmonary artery fibroblasts and stimulation of the mitogen-activated protein kinase (MAPK) family of signalling enzymes. Under the same conditions systemic artery fibroblasts were unaffected. This differential response of pulmonary fibroblasts to hypoxia represents a model to investigate the processes of pulmonary artery remodelling. The current study showed that acute hypoxia was capable of causing enhanced proliferation in pulmonary but not systemic artery fibroblasts and was linked to increased activation of p38 MAP kinase. Second, we have now shown that it is alpha and gamma isoforms of p38 MAP kinase, which are responsible. Third we have shown a link between stimulation of p38 MAP kinase and HIF-1 proportional, variant induction. An increased understanding of the effects of hypoxia on remodelling and proliferation represents a critical step in identifying targets for the treatment of pulmonary hypertension.

Hypoxia-driven proliferation of human pulmonary artery fibroblasts: cross-talk between HIF-1alpha and an autocrine angiotensin system

Pulmonary artery adventitial fibroblasts (FBPA) may play a central role in lung vascular remodeling under conditions of hypoxia and inflammation, the result being pulmonary hypertension and cor pulmonale. In cultured human FBPA, both angiotensin II (Ang II) and hypoxia promoted cell cycle progression and cell proliferation and suppressed apoptosis. These effects were further enhanced when both stimuli were applied simultaneously. Hypoxia elevated the expression of hypoxia-inducible factor 1alpha (HIF-1alpha) and increased the expression of genes regulated by the hypoxia-responsive element (HRE). Up-regulation of both angiotensin-converting enzyme (ACE) and Ang II receptor type 1 (AT1) was also observed. Exogenous Ang II further increased HIF/HRE-dependent signaling in FBPA, whereas suppression of the autocrine ACE-Ang II-AT1 loop with inhibitors of ACE, AT1, and phosphatidylinositol 3-kinase (PI3K) reduced the proliferative response to both hypoxia and exogenous Ang II. Overexpression of HIF-1alpha by transient transfection caused the same proliferative effect and up-regulation of AT1 expression that were observed under hypoxic conditions. In contrast, small interfering RNA targeting HIF-1alpha inhibited hypoxia-induced ACE and AT1 expression. Our studies indicate that the ACE-Ang II-AT1 system serves as a positive feedback loop and fosters FBPA proliferation under hypoxic conditions, with the PI3K-HIF-HRE axis as the central effector pathway. This pathway may thus facilitate vascular remodeling under hypoxic conditions.

Specific inhibition of p38 mitogen-activated protein kinase with FR167653 attenuates vascular proliferation in monocrotaline-induced pulmonary hypertension in rats

OBJECTIVES

p38 mitogen-activated protein kinase is associated with many clinical entities characterized by inflammation. We postulated that inhibition of p38 mitogen-activated protein kinase with FR167653 attenuates inflammation and the development of pulmonary hypertension in monocrotaline-treated rats.

METHODS

Rats were divided into 4 groups: (1) the control group (daily 0.9% saline), (2) the FR group (daily FR167653, 2 mg . kg(-1) . d(-1)), (3) the MCT group (daily 0.9% saline the day after a single monocrotaline dose, 60 mg/kg), and (4) the MCT+FR group (daily FR167653, 2 mg . kg(-1) . d(-1), the day after a single MCT dose). Body weight, pulmonary artery pressure, and morphometric changes of the pulmonary artery with the histopathologic method were observed weekly for 4 weeks. Also, p38 mitogen-activated protein kinase activity and inflammatory cytokine expression in the lung were measured.

RESULTS

Four weeks after monocrotaline administration, mean pulmonary artery pressure in the MCT+FR group was lower than in the MCT group (MCT+FR vs MCT: 24.7 +/- 1.9 vs 36.5 +/- 2.1 mm Hg; P < .05). In morphometric analysis the percentage of medial wall thickness and the percentage of muscularization in the MCT+FR group were reduced compared with those in the MCT group after 4 weeks (P < .05); however, the number of macrophages was not significantly different. p38 mitogen-activated protein kinase activity was significantly attenuated in the MCT+FR group compared with in the MCT group (7.2 +/- 0.52 vs 2.1 +/- 0.23 fold-increase, P < .05, at 1 week). Although mRNA levels of tumor necrosis factor alpha and interleukin 1beta were reduced in the MCT+FR group compared with in the MCT group (tumor necrosis factor alpha: 1.18 +/- 0.36 vs 3.05 +/- 1.12 fold-increase, P < .05, at 2 weeks; interleukin 1beta: 2.2 +/- 0.34 vs 4.4 +/- 1.09 fold-increase, P < .05, at 1 week), FR167653 did not suppress increased monocyte chemotactic protein 1 mRNA expression induced by monocrotaline (3.2 +/- 0.62 vs 3.1 +/- 0.42 fold-increase, at 1 week).

CONCLUSION

FR167653 significantly attenuates the expression of inflammatory cytokines, ultimately preventing the progression of pulmonary hypertension. These results suggest that p38 mitogen-activated protein kinase might play a central role in the molecular events that underlie the development and progression of pulmonary hypertension.